1. Field of the Invention
This invention relates to a carbon fiber with high performance characteristics and excellent thermal oxidation resistance produced from an acrylic fiber.
2. Description of the Prior Art
Carbon fibers have recently attracted attention as a reinforcing material for various composite materials due to their extremely high specific strength and specific modulus of elasticity, and have been employed in materials for aircraft and spacecraft, materials for sports equipment and materials for industrial uses. In addition, the characteristic properties of heat resistance, chemical resistance, abrasion resistance and electric conductivity as well as the above-described properties enable carbon fibers to be utilized for a wide variety of uses.
In using carbon fibers particularly for materials such as materials for high temperature furnaces, filter media, carbon fiber-reinforced plastics, carbon fiber-reinforced carbons, carbon fiber-reinforced metals, etc., oxidation resistance at high temperatures is a significant property when the molding steps and use conditions are taken into consideration.
Many techniques have so far been proposed including those disclosed in Japanese Patent Publication No. 4,405/62, and U.S. Pat. Nos. 3,285,696 and 3,412,062 for the production of carbon fibers. However, many commercially available carbon fibers have such poor thermal oxidation resistance that they are completely ashed by, for example, mere contact with air at 500.degree. C. for about 3 hours.
It has now been discovered that the thermal oxidation resistance can remarkably be improved if a phosphorus component and/or a boron component, and a zinc component and/or a calcium component is present in the carbon fiber in slight amounts.
An acrylic fiber containing phosphorus and sodium or potassium prepared by treating the acrylic fiber with a phosphorus compound and a sodium or potassium compound has heretofore been proposed for use as a starting material, thereby to facilitate preoxidation and carbonizing (e.g., as disclosed in Japanese Patent Publication No. 42,813/73 and British Pat. No. 1,214,807).
However, it has now been confirmed that the thus obtained carbon fiber containing phosphorus and an alkali metal such as sodium or potassium has an extremely low thermal oxidation resistance.